Preparation of tetrafluoroethylene



United States Patent 3,133,871 PREPARATION OF TETRAFLUOROETHYLENE William Rufus Von Tress, Lake Jackson, Tex assignor to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Jan. 11, 1963, Ser. No. 250,745 8 Claims. (Cl. 204-169) This invention pertains to a process for the preparation of unsaturated fiuorocarbons. More particularly, it pertains to a process for the preparation of tetrafiuoroethylene and other unsaturated fluorocarbons by subjecting a saturated fluorocarbon to the action of an abnormal glow discharge in the presence of carbon electrodes.

This application is a continuation-in-part of application 0 Serial No. 72,592, filed November 30, 1960, now

abandoned.

It is known that saturated fluorocarbons will react with carbon at temperatures in the range of 1700 to 4000 C. to produce unsaturated fiuorocarbons. The carbon arc has been suggested as a means of heating the fluorocarbon to this temperature. However, when a carbon arc is used, the conversion, for example, of carbon tetrafluoride to tetrafluoroethylene is relatively low. Further the high temperature is a disadvantage.

It is therefore an object of this invention to provide a process for the preparation of fluoro-olefins from saturated fluorocarbons employing carbon electrodes and lower temperatures. A further object is to provide a process for the preparation of the olefins from the saturated fluorocarbons whereby an improved yield is obtained. A still further object is to provide a process for the preparation of unsaturated fiuorocarbons by subjecting the fluorocarbon to the action of a particular electrical discharge.

The above and other objects are attained according to this invention, by passing, at a pressure in the range of 2 to 50 millimeters of mercury absolute, a saturated fluorocarbon having up to 2 carbon atoms between 2 carbon electrodes operating under suflicient voltage and limited electrode current density to subject the fluorocarbons to the action of the resulting abnormal glow discharge and reaction with the anode electrode to convert the saturated fluorocarbon to tetrafiuoroethylene and other unsaturated fluorocarbons, and separating the unsaturated fluorocar bon from the resulting product. By subjecting the saturated fluorocarbons to the action of an abnormal glow discharge using carbon electrodes decomposition of the saturated fluorocarbons and the formation of tetrafiuoroethylene and other unsaturated fluorocarbons by reaction with the carbon anode are obtained at temperatures in the range of 750 to 1250 C. In subjecting carbon tetrafluoride, for example, to the action of an abnormal glow discharge in the presence of carbon electrode at these relatively low temperatures, approximately 58 percent, for example, of the carbon tetrafluoride can be converted to tetrafluoroethylene per pass. This conversion is considerably greater than the 25 to 30 percent that can be normally expected when carbon tetrafluoride is converted to tetrafluoroethylene by subjecting the gas to the action of a carbon arc or high temperature. Operability of the instant novel process is assured by use of carbon electrodes which are consumed during the operation thereby acting as a source of carbon and actually taking part in the reaction. In the absence of a carbon anode or other source of additional carbon, CF, will not'convert to C E, at the reaction conditions of this process.

In electrical conduction and discharge through gases, a number of distinct stages or methods are encountered. Since all gases under reduced pressure contain a certain number of ions because of cosmic rays or other electromagnetic radiation present, a limited fiow of current may be obtained through a gas even though the gas is substantially non-ionized. This flow of current will be relatively independent of the voltage used. However, as the voltage is increased a point is reached often referred to as the breakdown voltage where the electrical potential is sufiicient to ionize the gas. Thus beyond the breakdown voltage the current is passed through the gas by the ionization of the gas. This is referred to as glow discharge. It gets its name from the soft luminous effect of the gas during this discharge. In glow discharge, a wide variation in electrode current density may be obtained without appreciable change in voltage. The voltage is nearly constant with current variation and it is necessary to limit or control the current desired by other external means.

If the current density in the glow discharge exceeds a certain maximum value for a particular gas, the characteristic of the discharge changes rapidly. An increase in voltage is necessary to increase the current. This new state is called the abnormal glow discharge. Any attempt to obtain a further increase in current during the abnormal glow discharge suddenly results in the glow discharge changing to an arc. In addition to a total difference in the physical appearance and the method of electrical discharge between the glow discharge and are discharge, the glow discharge is a low-current high-voltage discharge, while an arc is a high-current, low-voltage discharge.

It is surprising and unexpected to find that a saturated fluorocarbon, such as carbon tetrafluoride, hexafluoroethane, and mixtures thereof which are known to be generally stable compounds, decompose at pressures of 2 to 50 millimeters of mercury absolute, preferably 30 to 40 millimeters of mercury, to form unsaturated fluorocarbons at temperatures of 750 to 1250 C. when subjected to the action of the abnormal glow discharge therein produced in the presence of carbon electrodes. The contact time necessary to effect the conversion of the saturated fluorocarbons to an unsaturated fluorocarbon is a matter of a fraction of a second. The most convenient way to subject the fluorocarbon to the action of an abnormal glow discharge is to pass the gas through and between two tubular carbon electrodes which are operating under the proper conditions of voltage and electrode current density to obtain abnormal glow discharge within the fluorocarbon. For example in using tubular carbon electrodes having an inside diameter of approximately inch the fluorocarbon in the form of vapor or gas may be passed through the space between the electrodes at a rate of from to 1200 ml. per minute measured at standard conditions. At this rate the contact time of the fluorocarbon is from about 0.001 to 0.10 second.

While any voltage or current density at which abnormal glow discharge is obtained may be used to obtain the decomposition, an improved yield of the unsaturated 3 fluorocarbons is obtained when the voltage is above 400 volts preferably in the range of 600 to 900 volts. Voltages as high as 1200 volts may be used but no appreciable benefit is gained. The spacing of the electrodes is electrode having about a inch outside diameter and an internal through passage having a diameter of about /a inch. The current density was about 1 ampere/cmF, electrode spacing (distance apart) about 0.02 inch and such that generally the electrode current density is about 5 the voltage about 600 volts. 0.05 to 1 ampere per square centimeter. At a current The percent conversion of CR; reactant was about density in the range of 0.08 to 0.2 ampere per centimeter 60%. About 600 cc. of the product mixture was colsquare, an optimum yield is obtained. To obtain the lected in a cold trap and about 850 cc. in a receiver atabove conditions of current density at the desired volttached to the trap. Analysis of the product mixture in age, the spacing between the electrodes is usually in the the two vessels showed the following composition. range of /2 to 1 /2 inches on small electrodes and somewhat greater for larger diameter electrodes. Generally the spacing is of from 1 to 5 times the diameter of the electrodes. However, closer spacings can be satisfactorily Total Volume Percent employed. Once the abnormal glow discharge is effected g roduct the fluorocarbon gas can be continually passed between (00.) CF4 01F. CzFA 002 Sin the electrodes at the above conditions without the conduction through the gas changing to other forms. Higher 0010 Trap 000 30 4.2 01 4.2 0.5 electrical efficiency is obtained when the electrodes are Receiver 31 operated with a small spacing. However, when the distance is too small, for example A or less, it is some what difficult to maintain the abnormal glow discharge. There is a tendency for the conduction through the gas substantlally Pure a used as a feed T1115 to convert to an are which results in an immediate temhas EKG/F of about The P perature rise. At abnormal glow discharge, a tempera- 25 uct mlxture fnchldmg unreacted Q madam, has ture in the range f 5 to 1250 C. is Obtained C/F atom ratio of about 0.3 9. This increase in carbon The following examples will serve further to illustrate 1s attF1buted to carbon supPlmd by the anode durmg the the present invention but are not meant to limit it thereto. reactlon- Example 3 Example I A reactor was constructed using copper holders for A further l d in an apparatus and under tubular carbon electrodes The cylindrical copper 1 reactionconditions similar to that described in Example trode holders were water cooled. The gas being charged AS 1n EXample 2, a materlal balance Was made for to the reactor was passed through one of the copper the reactants charged and Products Produced y reacting holders, one of the tubular carbon electrodes, between 4 in the glow discharge pp the gap between the two electrodes and then out of the For this R1 was charged at a flow rate of about reactor through the second electrode. A sight glass was 188 mint-1w for 10 minutes through a tubular Carbon provided f th reactor 50 th t th type f l t i l electrode having about a inch outside diameter and discharge obtained between the electrodes could be noted. an internal through Passage having a diameter of about Two different sized electrodes were used. One of the 40 1/8 inchcarbon electrodes used had a inch outside diameter The current density was about 1 ampere/cmP, elecand an inside diameter of A5 inch. The larger sized trode spacing (distance apart) about /8 inch and the electrodes had a /8 inch outside diameter and had a pasvoltage about 750 volts. The percent conversion of CR; sageway through the center of inch. reactant was about 40%. About 580 cc. of product mix- T he pertinent details, and the results obtained are given ture was collected in a cold trap and about 1000 cc. in in the table below: a receiver attached to the trap. Vapor phase chromato- Rate 01' Percent Electrode Tempcra- CF; Conver- Outside Active Current Spacing 0i ture of Gas Charged, sion of Run No. Diameter, Electrode Density, Electrodes, Volts Between mL/Min. CF; to

Inch Area, 0111. 7 Amp/cm. 1 Inch Electrodes, at Stand- 0113; Per

C. ard 0011- Pass ditions 0 25 0.08 3% 900 1,125 308 52 at 15 0. 09 ,0 540 1,000 188 52 as 15 0. 10 1 580 1, 000 65 7s 30 0.120 3%. 925 1,200 472 50 The temperature of the gas as it flowed between the graphic analysis of the product mixture in the two vessels electrodes was determined by means of an optical indicated the following composition. pyrometer.

When copper electrodes were substituted for the carbon anode and cathode, there was substantially no conversion Total Volume Percent Volume of of a- Product Similar results are obtained when hexafluoroethane is (m) CF4 CzFo @213 used in place of carbon tetrafluoride.

Cold Trap 580 62.5 5.2 32.2 Example 2 R eiv 1,000 73. 1 3. 2 24.6

In a second test made in an apparatus and under reaction conditions similar to that described in Example 1, 7 As in Example 2, substantially pure CE; was used as a material balance was run for the reactants charged and a feed gas. This material has a C/F atom ratio of about products produced by reacting CE; in the glow discharge 0.25. The product mixture, including unreacted CF reapparatus, actant, has a C/F atom ratio of about 0.32. The calcu- For this run, C1 was charged at a flow rate of about lated Weight of carbon fed into the reactor was 0.92 gram. 65 cc./minute for 25 minutes through a tubular carbon The calculated weight of carbon in the product was 1.02

grams. Again, the increase in carbon is attributed to carbon supplied by the anode during the reaction.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that I limit myself only as defined in the appended claims.

I claim:

1. A process for the preparation of an unsaturated fluorocarbon, which comprises passing, at a pressure in the range of 2 to 50 millimeters of mercury absolute, a saturated perfluorocarbon having up to 2 carbon atoms between two carbon electrodes operating at a voltage and anode current density to thereby obtain abnormal glow discharge through the saturated perfiuorocarbon to convert said saturated perfluorocarbon to unsaturated fluorocarbon, and separating the unsaturated fluorocarbon from the resulting product.

2. A process for the preparation of the unsaturated fluorocarbons, which comprises passing, at a pressure in the range of 2 to 50 millimeters of mercury absolute, a saturated perfiuorocarbon having up to 2 carbon atoms between two carbon electrodes operating at a voltage of 400 to 1200 volts, a current density in the range of 0.05 to 1 ampere per square centimeter, and at a temperature in the range of 750 to 1250 C. to thereby obtain an abnormal glow discharge through said saturated perfluorocarbon between the two carbon electrodes to convert the saturated fluorocarbon to unsaturated fluorocarbons, and separating the unsaturated fluorocarbons from the resulting product.

3. A process according to claim 2 wherein the perfiuorocarbon is carbon tetrafluoride.

4. A process according to claim 3 wherein the perfiuorocarbon is hexafluoroethaue.

5. A process for the preparation of the tetrafluoroethylene, which comprises passing between two carbon electrodes, at a pressure in the range of 30 to millimeters of mercury absolute, carbon tetrafluoride at a voltage of 600 to 900 volts, a current density in the range of 0.05 to l ampere per square centimeter, and at a temperature in the range of 750 to 1250" C. to thereby obtain an abnormal glow discharge through the carbon tetrafluoride between the two carbon electrodes to convert the carbon tetrafluoride to tetrafluoroethylene, and separating the tetrafluoroethylene from the resulting product.

6. A process according to claim 5 wherein the electrode current density is in the range of 0.08 to 0.2 ampere per centimeter squared.

7. A process for the preparation of the tetrafluoroethylene, which comprises passing, at a pressure in the range of 30 to 40 millimeters of mercury absolute, hexafluoroethane between two carbon electrodes operating at a voltage of 600 to 900 volts, a current density in the range of 0.05 to 1 ampere per square centimeter, and at a temperature in the range of 750 to 1250" C. to thereby obtain an abnormal glow discharge through said fluorocarbon between the two carbon electrodes to convert the fluorocarbon to tetrafluoroethylene, and separating the tetrafluoroethylene from the resulting product.

8. A process according to claim 7 wherein the electrode current density is in the range of 0.08 to 0.2 ampere per square centimeter.

References Cited in the file of this patent UNITED STATES PATENTS 2,709,192 Farlow May 24, 1955 

1. A PROCESS FOR THE PREPARATION OF AN UNSATURATED FLUOROCARBON, WHICH COMPRISES PASSING, AT A PRESSURE IN THE RANGE OF 2 TO 50 MILLIMETERS OF MERCURY ABSOLUTE, A SATURATED PERFLUOROCARBON HAVING UP TO 2 CARBON ATOMS BETWEEN TWO CARBON ELECTRODES OPERATING AT A VOLTAGE AND ANODE CURRENT DENSITY TO THEREBY OBTAIN ABNORMAL GLOW DISCHARGE THROUGH THE SATURATED PERFLUOROCARBON TO CONVERT SAID SATURATED PERFLUOROCARBON TO UNSATURATED FLUOROCARBON, AND SEPARATING THE UNSATURATED FLUOROCARBON FROM THE RESULTING PRODUCT. 